1. Field of the Invention
The present invention relates to a high toughness, high strength steel having excellent resistance to stress corrosion cracking in a stress-corrosive environment such as seawater or salt water.
2. Description of the Related Art
The growing energy demand of recent years has led to a search for new energy supplies and thus to interest in development of seabed resources and geological surveys of the seabed. This has also stimulated the construction of marine structures and seabed research vessels as well as plans for construction of seabed bases for oil production, etc.
These structures must be free from distortion, failure, etc. caused by wave motion and water pressure and must be of a higher level of safety. Therefore, it is required that materials used for such marine or seabed structures have high weldability, high strength, and high toughness and further have high resistance to stress corrosion cracking under service conditions such as seawater.
Most steel plates used for these structures have a large thickness (t). It is desired that they satisfy specific requirements such as ASTM A20 (General Requirements for Steel Plate for Pressure Vessels), 11, 5, 3, and 12, 1, 4, which oblige that a required value be satisfied at not only the 1/4 t position but also the surface portion (test pieces with central axis of position 7 mm from steel plate surface) and the center portion (1/2 t).
To meet the need for a safer, more reliable steel, development has been made of high strength, low alloy, Ni-containing steels and processes for producing the same. Typical ones are disclosed by U.S. patent application Ser. No. 798,870 to the same assignee, U.S. Pat. No. 4,572,748, and Japanese Unexamined Patent Publication (Kokai) No. 61-272316.
These all use so-called direct quenching in which a steel plate is water-cooled directly after hot rolling. A direct-quenched steel exhibits a higher hardenability than that of a reheat-quenched steel and provides a higher strength but has a poorer toughness.
In U.S. patent application No. 798,870 a steel slab is heated at a very low temperature of 900.degree. C. to 1000.degree. C. and subjected to low temperature hot rolling, then direct quenching followed by tempering, with the result that the effective grain size is refined to obtain a high toughness steel having a high brittle crack arresting capability never achieved by conventional steels. In U.S. Pat. No. 4,572,748, a steel plate is imparted with uniform mechanical properties by suppressing fluctuation along the length by simultaneously cooling the entire steel plate and by suppressing fluctuation along the thickness by controlling the water flow density to minimize the cooling rate difference between the surface and the inside of the steel plate. However, these do not consider the stress corrosion in an environment where contact with salt water occurs, e.g., the stress corrosion of marine structures in seawater, and cannot ensure full safety for marine use.
Japanese Unexamined Patent Publication (Kokai) No. 61-272316 discloses a process for producing steel having good resistance to stress corrosion cracking in seawater, wherein a Ni-containing steel with an Nb additive and having a reduced amount of the impurity elements P, N, and O is hot-rolled and subjected to direct quenching and tempering under appropriate conditions.
To study the stress corrosion cracking of high strength steels, the linear fracture mechanics theory, which employs the stress intensity factor, K value, at the crack tip, has been applied to quantize the fracture behavior of cracks or defects inherent in the material under corrosive environments. A pre-cracked specimen is subjected to a stress corrosion cracking test under an service environment to establish a severe condition at the notch root and accelerate the initiation of delayed failure. A set of dead-weight tests at various K levels is performed under the above condition to obtain K.sub.ISCC value (Mode I fracture occurring under plain strain condition), a critical and constant K value at and below which no delayed fracture takes place any longer. The K.sub.ISCC value is employed as a criterion to estimate the resistance to stress corrosion cracking.
The above Japanese publication describes an improved K.sub.ISCC value of the welding heat affected zone (HAZ) in seawater, which reaches 450 kgf.multidot.mm.sup.-3/2 at highest but is not sufficiently high.